A patient weighing scale of the general type as the present invention is disclosed in the copending application, Ser. No. 664,709, filed Mar. 8, 1976. The scale disclosed in the copending application comprises a conventional frame structure with a cantilever boom having a free end as well as a secured end pivoted to the frame structure. A stretcher arrangement is removably and pivotally connected to the boom by a support which includes two bending moment measuring transducers responsive to the weight on the stretcher. The outer ends of the transducers are fixed to hoops which are in turn secured to the stretcher proper. The stretcher proper is thus fixed relative to the transducers. As a result, if the patient's weight is not perfectly aligned along a center line extending longitudinally down the middle of the stretcher, the overall stretcher as well as the transducers will rotate somewhat away from a vertical plane until the patient's center of gravity is directly beneath the free end of the boom. Transducers of this type measure bending moments in only a single plane which in this case would extend through the center line of the stretcher. Accordingly, when the stretcher and transducers are rotated away from the vertical, they will respond to only a fraction of the patient's weight, thereby providing an erroneous reading.
Patient weight scales are often used to measure the weight of very ill and often highly dehydrated and underweight persons such as burn patients. It is very important in monitoring such patients' progress to measure very small increases and decreases in weight with great accuracy a number of times in a single day. Consequently, very small errors in weight change measurement can be very significant. It is therefore desirable to improve the accuracy of patient weighing scales to the maximum extent possible.
The present invention can significantly improve the reliability of patient weight measurements. The invention consists of an improved design of the transducer means and the support element connecting the stretcher to the cantilever boom of the frame structure in a patient weighing arrangement such as is disclosed in the copending application, Ser. No. 664,709, the components of which are summarized above.
In the preferred embodiment, a conventional elongated transducer means which measures bending moments at both of its ends is mounted in an open ended tube housing. The housing is removably and pivotably mounted at its top surface to the free end of the cantilever boom. The outer ends of the transducer means are fixed to end plates which in turn pivotally support therebelow a longitudinally extending support bar. The support bar supports the stretcher beneath. This structure operates to permit the transducer means to remain in the vertical plane regardless of the location of the center of gravity of the patient relative to the stretcher on which he lies prone, thereby insuring the accuracy of the weight measurement.
The arrangement has two pivotal axes, one above the load cell at the boom, another between the side plates beneath the load cell. In accordance with another feature of the invention, these two pivotal axes are equidistant from the load cell's center axis, which provides fast acting self-righting moment forces which keeps the load cell vertical regardless of the elevation of the boom or the location of the center of gravity of the patient.
In accordance with another aspect to the invention, the transducer means may comprise a single elongated double ended load cell, rather than two separate load cells as used in prior patient scales. Each end of the load cell can be provided with either a full bridge or half bridge circuit arrangement of strain gauges. The two ends may be matched by mechanical fine tuning instead of electrical matching as is required with two separate load cells.
Two pivot points, both beneath the weight measuring transducers, have been used in a traveling block weighing device disclosed in U.S. Pat. No. 3,095,057 issued to Kraeling. However, Kraeling's device is designed for protection against dangerous side loads on its two vertical tension type load cells, produces much reduced self-righting forces and would not be useful in a patient weighing scale designed for enhanced accuracy and reliability.